The present disclosure relates to a method for producing a substantially single mode laser pulses and a laser for producing the same.
When a beam of light enters the eye, the beam passes through the cornea, lens, and the vitreous humor. The portion of the beam that is not absorbed is focused by the eye lens onto the retina. Under normal conditions, the light energy is converted by the retina into chemical energy, stimulating optical sensations. Laser beams with wavelengths longer than about 2.2 μm are strongly absorbed by the cornea and can cause damage to the cornea. Laser beams with wavelengths shorter than about 1.4 μm are not absorbed in the cornea or vitreous humor and therefore can cause damage to the retina. Laser beams having a wavelength in the range of 1.5 μm to 2.2 μm are not absorbed by the cornea, but are completely absorbed by the vitreous humor of the eye thereby alleviating any damage to the retina. Therefore, laser beams having a wavelength in the range of 1.5 μm to 2.2 μm are generally considered to be “eye-safe” lasers.
It is always desirable to have a powerful laser beam while still maintaining the quality of such laser beam. Beam quality is typically measured by how fast a laser beam grows in size as it propagates along, relative to that of an ideal beam. Examples of ideal beams are top hat and gaussian beams. An ideal top hat beam starts out with a uniform intensity across an aperture and a flat phase front. A gaussian beam has a bell-shaped intensity profile and a flat phase front. Another example of an ideal beam is the lowest-order mode of an optical fiber. A diffraction-limited laser beam is one that grows in size at a rate equal to that of an ideal beam.
Multi-mode fibers are optical fibers with a relatively large core area that can support multiple propagation modes. In contrast, a single-mode fiber has a substantially smaller core area and is able to support only one propagation mode. Typical core diameters for single-mode fibers are around 9 microns, whereas the core diameters of multi-mode fibers can reach hundreds of microns.
Q-switched pulses are energetic pulses produced by lasers by rapidly switching the Q factor (or quality factor) of a laser resonator from low to high. A high Q factor corresponds to low resonator losses per round trip, and a low Q factor to high round trip losses. This is normally accomplished using a variable attenuator inside the resonator cavity. In passively Q-switched lasers, the variable attenuator is a saturable absorber. A saturable absorber has a low transmittance (high loss) initially, but quickly increases its transmittance (low loss) when the intensity in the resonator cavity reaches a high enough level. Q-switched laser pulses can range from tens of picoseconds to hundreds of nanoseconds, with peak powers ranging from kilowatts to Gigawatts.
Microchip lasers do not employ pump guiding and are therefore limited to short cavity lengths to achieve sufficient coupling between diode pump and laser signal beams due to faster spreading (lower beam quality) of diode pump light. The short microchip cavity results in a correspondingly small lowest-order transverse laser mode diameter. Prior attempts to achieve a larger signal beam diameter in a microchip laser by increasing the pump beam diameter, resulted in the excitation of multiple signal transverse modes. This causes an increase in laser pulse width or multiple pulsing, and a degradation in signal beam quality. In eye-safe microchip lasers (i.e., microchip lasers having an eye-safe wavelength radiation), Erbium ion concentration is limited by up conversion effects, thus further limiting the achievable gain. For all these reasons, the output pulse energy of a sub-nanosecond microchip laser is typically only a few microjoules (at best). Generation of nanosecond eye-safe laser pulses with an optical parametric oscillator (OPO) is problematic. This is due to the fact that a long pump pulse is required for the signal beam to build-up from noise in the OPO cavity. In an OPO, the pump energy is stored in the electromagnetic field of the pump beam, and not in an excited state of a laser ion such as Erbium. This means the OPO signal beam will have a similarly long pulse width. An optical parametric amplifier (OPA) has been used to generate short 1.5 μm pulses but it requires a sub-nanosecond, high pulse energy pump laser and many passes through an OPA crystal. Typical conversion efficiency from pump to signal is only 25-30%. This OPA architecture is significantly larger and more complex than a microchip laser. Also, using this as a master oscillator (MO) seed for an Erbium-doped amplifier is difficult since its wavelength must be closely matched to that of the amplifier. Mode-locked Erbium lasers can also be used to generate sub-nanosecond pulses at 1.5 μm, but are extremely large and complex.
The present disclosure provides improvements over the prior art methods for producing a substantially single mode laser pulses.